Field Note: Possible Cause of Mid-Canopy Wood Peeling in Almond

Cameron Zuber, Staff Research Associate, Merced County

Over the years working in almond orchards, I have noticed the occasional occurrence of branches with wood peeling. It only seems to affect a small portion of the branch (e.g., one to ten inches long) and was in the mid-canopy. Fresh peeling was typically seen in the fall and winter periods on almond trees old enough to be harvested. The peeling exposed a large area of the inner wood and there were concerns this could be a location of infection especially since it was seen in the wetter seasons. The challenge was determining the cause of this peeling and once identified if there was a way to prevent or mitigate the damage. Recently observed peeling may have provided some indication of a possible cause.

In the beginning of September, fresh peeling was seen on a third-leaf almond orchard in Fresno county with peeling ranging from the bark to sapwood layers. The peeling occurred on about a fourth of the trees on the outer and inner canopy branches, though the outer branches seem to have a higher degree of damage.

Photos of third-leaf ‘Nonpareil’ almond tree branches with peeling wood at beginning of September. Gloved hand behind branch for scale.

This damage was thought to be new because this orchard is visited once to twice a week to collect information for almond farming research. Visits required walking the entire site and working within the tree canopy, so there was high confidence the peeling was not present until it was observed in September. The perplexing thing was recently no practice had occurred that would explain this type of damage, like a spray boom running through the tree canopy during a pesticide application.

In fact, the only recent event was harvest which did not immediately explain the wood peeling. A shaker could damage the main trunk but not the branches, and the likelihood that branch-to-branch contact during a shake would cause something like this was low as the shaker tends to vibrate the branches not sway them. Perhaps a harvest or sweeper operator may have allowed the equipment to drive through the canopy of the trees. However, this equipment was not used as the decisions was made to pole the site with a small crew using metal poles and tarps to collect the nuts.

Photo of third-leaf ‘Nonpareil’ almond being harvest at the end of August using poles and tarps.

Pole harvest may seem odd, but there were two reasons for this. First, these were younger trees and there were concerns of excessive shaker damage. Second, there was a need to selectively collect nuts from specific sets of trees for almond farming research. Poling addressed these needs and concerns. Given the timing of the pole harvest and peeling damage this was the most likely cause. The height of the peeling also lined up with the main area the poling crew was hitting trees the hardest. Nonetheless, a possible alternative was also considered.

Ties were used to support the young tree canopy as the fruit nuts developed. This prevented branches from breaking from the fruit nut’s weight. Rubbing from the ties as the trees swaying in the wind may have resulted in this peeling damage. However, little of the damage occurred near the ties and the most severe damage occurred on the outer canopy while the ties were located on the inner canopy.

Photo of third-leaf ‘Nonpareil’ almond with red line denoting approximate height seen for wood peeling and location of ties (left) and wood peeling on the outer canopy with orange tie on inner canopy (right).

The reason for the peeling may be due to the energy from a hard-hitting pole causing fractures to form parallel to wood fibers. Tension from the undamaged fibers pulled this fracture so an opening formed thus exposing the inner layers of wood.

If the pole harvesting caused this peeling, it provides insight into the fresh peeling seen at other locations during the fall and winter since poling is used to perform sanitation. No matter how hard or efficient the harvesting effort, there are always a few nuts left in the trees afterwards. These nuts may allow navel orangeworm (Amyelois transitella) larvae to overwinter contributing to the first adult generation during the following growing season. Knocking these nuts off the trees increase the chance the larvae will perish before becoming adults. Hitting the trees with poles or shaking the trees is a way to knock the nuts off the trees in the fall or winter to help sanitize the orchard.

Unfortunately, the wood peeling possibly caused by the pole harvest in the summer does not tell whether pole sanitation in the fall or winter would result in the same type or amount of damage. The high amount of peeling from pole harvesting a third-leaf orchard was probably due to the suppleness of the wood due to the trees’ age, irrigation making the wood swell, or combination increasing the chance of damage from a pole. Whether older and dormant almond trees in the fall or winter have the same suppleness is unknown. It is also unknown if this type of damage increases infection from diseases and therefore it cannot be said if this is a big enough concern to try and prevent within almond production.

Ultimately, you manage your trees. If you think this peeling is a large enough concern, then take proper mitigation measures. This could include properly instructing the poling crew or limiting poles to kinds that would reduce the likelihood of damage, like using plastic or bamboo instead of metal poles. Balancing preventative measures and efficiency is something that needs to be figured out in the field as the technique and vigor of a swing can be robust depending on the individual.

Special thanks to David Doll (former Farm Advisor) for first mentioning this type of damage to me and assistance with determining the possible cause for these recent observations, Brent Holtz (Farm Advisor with UC Cooperative Extension in San Joaquin County) for his continued support of the research resulting from these observations, and Vincent Silva (Superintendent of Agriculture) for reviewing this document and providing edits.

Field Note is a series of posts that will cover observations made from the field by staff research assistants of the University of California Cooperative Extension office of Merced County. While Field Note posts may reference completed or on-going research, its intent is share notable observations from the field, not to summarize research findings or describe current recommended farming practices. Field Note series was started at The Almond Doctor website, but now is posted to other resources.